a. Field of the Invention
The present invention relates to a device for detecting an offset angle (tracking angle) of a pickup arm in a record player, a video disc player or the like. More particularly, it pertains to a device for detecting an offset angle of a pickup arm which does not carry on the arm support mechanism any such additional heavy detecting means as will cause a loss of the lateral balance of the pickup arm.
B. Brief Description of the Prior Art
The signal-carrying groove of a phonographic disc is engraved or cut by a cutter which moves constantly linearly in a radial direction of the plane of the disc above thereof which is to be cut. In case the disc which carries a signal groove is played on for example a record player employing a conventional swingable and pivotable pickup arm, the reproduction of the recorded signal is conducted by a stylus-carrying cartridge which is mounted at the foremost end of the pickup arm of a certain length which moves forming a circular arcuate loci relative to a fulcrum of the pickup arm swingably and pivotally supported on a support mechanism. Therefore, in case the stylus which is carried on the foremost end of the pickup arm is set initially at the start of play so that its facing direction is held exactly keeping a tangential line relative to the beginning portion of the groove of the phonographic disc, it will be understood that as the tip of the stylus travels farther forward toward the center of the disc, the direction of facing of the stylus will change progressively and accordingly there will progressively arise a tracking error of the stylus. Conversely, in the similar manner, in case the stylus carried on the foremost end of the pickup arm is set so as to produce no tracking error (offset angle) .DELTA..phi. when it is positioned at the innermost groove close to the center of the grooved disc, a tracking error will develop most intensively at the starting part of the groove engaged by the tip of the stylus, gradually lessening the magnitude of this tracking error as it travels progressively toward the center of the disc. As such, a second higher harmonic distortion will arise in each of the above-mentioned cases, resulting in a loss of fidelity in the reproduction of the original signal to be recorded.
In order to minimize the development of such tracking errors, there has been used in the past a pickup arm which is bent, when manufactured, to have an offset angle .DELTA..phi. at an appropriate portion of its whole length. With such an off-set pickup arm, the problem of development of the horizontal tracking angle deflection is solved to a certain extent. However, there will be exerted on the pickup arm a force which tends to urge the pickup arm to swing clockwise relative to its fulcrum. Such a force may be called an inside force. This force will act so as to push the reproduction stylus toward the inside of the walls constituting a signal groove of the disc as this disc is being played, or it will act as a force to destroy the lateral balance of the running pickup arm. Thus, such a force imparts the pickup arm an adverse effect on the ability of this pickup arm to trace the signal groove.
The occurrence of the above-said inside force may be reduced to some extent by the additional provision of an inside force cancelling means on the pickup arm supporting mechanism. Also, the development of the aforesaid lateral imbalance of the pickup arm may be diminished to some extent by the additional provision of a lateral balance keeping means on the pickup arm support. Nevertheless, these prior attempts have the drawbacks such that only one side of the walls of the signal groove of the phonographic disc tends to wear quicker than does the other side of the walls, or only one side of the reproduction stylus tends to wear as a disc is played frequently on the conventional record player.
In order to eliminate the foregoing drawbacks of the prior art, there has been proposed a linear tracking pickup arm system or the tracking error-less pickup arm system which is designed so that the pickup arm slides linearly radially toward the center of the phonographic disc during play while keeping a tangential line relative to the signal groove of the phonographic disc.
With this known linear tracking pickup arm system, however, it is practically impossible to always keep the pickup arm tangential to the circumference of the phonographic groove of the disc on account of a deviation or vibrations which the pickup arm undergoes at the time of reproduction, due to the errors in mechanical movement transmitting mechanism of the system which are introduced at the time of manufacture. In order to eliminate such deviation of the tracking angle which takes place due to the failure of keeping the pickup arm tangential relative to the signal groove of the disc, it has been proposed to employ additionally a tracking angle detecting and compensating means such as a light-to-electricity (photo-electric) transducer to detect the tracking angle of the pickup arm, and also a servo motor connected to the pickup arm driving means so that, upon a deviation of the pickup arm is detected, a voltage which is proportional to the detected lateral deviation of the pickup arm is applied to this servo motor to move the pickup arm in the compensating direction and distance so as to automatically render the deviation output or the tracking error of the pickup arm zero.
As the tracking angle detecting and compensating means of the prior art, there has been employed an arrangement such that a shutter plate having a narrow slit formed therethrough is provided on the pickup arm supporting mechanism which is assigned to cause horizontal swinging movement of the pickup arm, so that any variation of the amount of light rays which are allowed to pass through this slit is detected so that, accordingly, the trifle change representing a horizontal deviation of the pickup arm is detected. The tracking error detecting means of the prior art invariably is arranged so that the aforesaid slit-carrying shutter plate is additionally attached to the horizontal swingable shaft of the pickup arm supporting mechanism such as a gimbal support mechanism in such a manner that this attachment of the shutter plate unavoidably serves to add a certain amount of eccentric load on said shaft of the pickup arm supporting mechanism. Thus, the known technique results in a major cause for a loss of lateral balance of the pickup arm due to the weight of such an accessary. Besides, the known tracking error detecting and compensating technique requires a complicated arrangement. As such, its manufacturing cost increases undesirably.